So who decides someone is transgender? If it has nothing to do with the chromosomes or secondary sexual chacteristics, then what is the basis for this medical diagnosis?

For example a person who is bipolar or schizophrenic... this is a psychiatric disorder... but there is a strong genetic link so there is definitely a biological basis that causes it, not necessarily environment. A schitzophrenic or bipolar person doesn't choose to be so. So I'm not saying that there is not a biological link regarding a "transgender" person. But they are still psychiatric illnesses.

"Gender identity disorder*(GID), also known as gender dysphoria, is a formal diagnosis used bypsychologists*and*physicians*to describe people who experience significant*dysphoria*(discontent) with the*sex*they were*assigned at birth*and/or thegender roles*associated with that sex. Affected individuals are commonly referred to as*transsexual or*transgender."

And male and female are not just human characteristics. They are characteristics of every animal in nature that reproduces using sexual reproduction.